LASER SPECTROSCOPY OF ALEXANDRITE CRYSTALS : FOUR-WAVE MIXING, TWO-PHOTON ABSORPTION, AND TIME-RESOLVED SITE-SELECTION

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LASER SPECTROSCOPY OF ALEXANDRITE

CRYSTALS : FOUR-WAVE MIXING, TWO-PHOTON

ABSORPTION, AND TIME-RESOLVED

SITE-SELECTION

R. Powell

To cite this version:

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Colloque C7, suppl6ment au nOIO, Tome 46, octobre 1985 page C7-403

LASER SPECTROSCOPY OF ALEXANDRITE CRYSTALS

:

FOUR-WAVE MIXING, TWO-PHOTON ABSORPTION, AND TIME-RESOLVED SITE-SELECTION

R.C. Powell

Physics Department, Oklahoma State University, StilZwater, Oklahoma

74078,

U.S.A.

~ 6 s u m d

-

Une Gtude detaillbe des propri656s optiques de

monocristaux

d'alesandrite a 6td rialisie au moyen de

diffgrentes techniques de spectroscopic laser.

La

spertros-

copie

a'

sglection de site r6solue danrj le temps a' l'aide d'un laser

d

colorant accordable, a i t @ utilisde

~t

a

permis de mettre en &idenre six types de paires d'ions Cr3) &change couplc dans le rsseau cristallin. L'excitation par impulsicn au picoseconde d e haute puissance a dt6 utilisge afin de

produire une absorption deux photons et le spectre

d '&mission qui en rLsr.11 te laisse appara?tre une nouvclle bande de f luorcscence au v ~ i s i n a g e de 400 nm.

La

technique des quatre-ondes ndl anrj6es a i t

6

empl:zyie pocr gtabl

i

r et explorer l a "grille" (grating) de

population

de 1'Gta.t excitg d e c ions

~ 1 - ~ ' * dans les daux types de sites. Le5 r & ~ l t a t & de cea

exp&rienes faurnissent d'importantes inforaatio?~ c ~ n c e r n a n t I'utilisatioq d r l'ale:<a~drite en tant que matbriau salidc pour des lasers accord able^.

Rhstract - Details o,f

the

o p t i c a l spmctrnscopic p r o p e r t i n s a < alexandrite crystals were studied hy di f4arent laser spectros- copy techniques. Time-resolved site-selectian spertroscapy with a tunable dye laser was 115ed to identify six t,ypas o f

~ x c h a n g e coupled pairs of cr3" i c n s in

the

lattice. High-. power, picosecond pulse excitation was used t o produce two- photon absorption and the resalting emission apectrtrm w a r ,

found to exhibit a new fluorescence band in the

400

nm spectral region. Four-wave mixing technj.q:lec, w e r e xsed

t=,

establish and probe excited stst.e population yrat ings

indapendently for cr3+ ions in rnirrar and inversion sites.

The

result5 nf these experiments provide important information c:ancerning t h r ;

use

o,f a1s:tandrite as a tanable sclid f;ta+c laser materi dl.

Alexandrite (12e41204: rr3'! is important as a tunable sol id state laser material /I/.

The

characterictics of the optical spectra provide important information for understanding the fundamental physical pro-

cesses relevant t o the use of this material in lasers. The basic spectroscopic properties of a1e:candrit.e have been reportcd previour,I.y /1,2/, (Powell, Xi, Gang, Buarlea, and Walling, to be published in Physical Review

B).

We

summarize here results obtained using several types of laser spectroscopy measurements which provide

new

i.nfarn~tion concerning the properties of this material.

The sample used for this investigation was an oriented, single crystal

C7-404

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cube o f a l e x a n d r i t e w i t h each edge about 5 mm i n l e n g t h , c o n t a i n i n g

0.0897 a t . %

cr3+

ions. The M3+ i o n s i n t h i s l a t t i c e occupy two

i n e q u i v a l e n t c r y s t a l f i e l d s i t e s h a v i n g m i r r o r symmetry or i n v e r s i o n symmetry. The

cr3+

i o n s e n t e r t h e c r y s t a l s u b s t i t u t i o n a l l y f o r t h e A13+ i o n s , 78% r e p l a c i n g A13+ i o n s i n t h e m i r r o r s i g h t s and t h e r e s t

going i n t o t h e i n v e r s i o n s i t e s . Lasing occurs o n l y w i t h m i r r o r c i t e

cry+

ions.

The a b s o r p t i o n spectrum i s dominated by t r a n s i t i o n s associated w i t h m i r r o r s i t e ions. The major f e a t u r e s c o n s i s t o f two brna.d bands

centered at about 420 and 580 nm, t h r e e s e t t i of sharp l i n e s centered

near 680, 650, and 470 nm, and t h e band edge appearing below 220 nm. T h i s i s s i m i l a r t o

cr3+

s p e c t r a observed i n o t h e r h o s t c r y s t a l s . E x c i t a t i o n s p e c t r a show t h e a b s o r p t i o n bands o f i o n s i n t h e i n v e r s i o n s i t e t o be s h i f t e d t o s i g n i f i c a n t l y h i g h e r energy compared t o t h e same t r a n s i t i o n s o f i o n s i n t h e m i r r o r s i t e s .

The major f e a t u r e s o f t h e f l u o r e s c e n c e spectrum a r e t h e two sharp

l i n e s RmI and RZm near 680 nm and t h e broad, s t r u c t u r e d band peaking

a t lower energies. The l a t t e r i s due t h e s u p e r p o s i t i o n o f low energy v i b r o n i c emission t r a n s i t i o n s from t h e components o f t h e 2~ l e v e l and emission from t h e r e l a x e d e x c i t e d s t a t e o f t h e 4 ~ 2 g l e v e l waich i s Stokes s h i f t e d t o lower energy compared t o t h e a b s o r p t i o n t r a n s i t i o n i n v o l v i n g t h i s l e v e l . Far c e r t a i n e x c i t a t i o n wavelengths, t h e Rli and HZi l i n e s a s s o c i a t e d w i t h

cr3+

i o n s i n i n v e r s i o n s i t e s can be seen a t

lower energies than t h e m i r r o r s i t e R-lines. Measurements o f t h e

temperature dependences o f t h e f l u o r e s c ~ n c e l i f e t i m e s and l i n e w k d t h s o f t h e R - l i n e s show t h a t d i r e c t phonon a b s o r p t i o n processes dominate these s p e c t r a l p r o p e r t i e s o f i o n s i n m i r r o r s i t e s w h i l e Raman s c a t - t e r i n g o f phonons i s t h e dominant l i n e b r o a d e n i n g processes f o r i n v e r s i o n s i t e i o n s (Powell, X i , Gang, BuarXes, and Walling, t o be p u b l i s h e d i n P h y s i c a l Review

B).

The l a t t e r e x h i b i t a temperature independent l i f e t i m e ,

I 1

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TWO-PHOTON ABSORPTION

The f l u o r e s c e n c e emission o f a l e x a n d r i t e was observed a f t e r two-photon a b s o r p t i o n e x c i t a t i o n . A 30 p s p u l s e from a frequency doubled Nd-YAG l a s e r a t 532 nm p o l a r i z e d p a r a l l e l t o t h e b d i r e c t i o n was used f o r e x c i t a t i o n . T h i s has weak single-photon a b s o r p t i o n by m i r r o r s i t e i o n s i n t h e v i s i b l e s p e c t r a l r e g i o n and s t r o n g two-photon a b s o r p t i o n i n t h e U.V. r e g i o n t e r m i n a t i n g on a shoulder o f t h e band edge near 266 nm. The f l u o r e s c e n c e observed a f t e r t h i s t y p e o f e x c i t a t i o n occurs i n two regions: t h e normal emission i n t h e 700 nm r e g i o n and a new

emission band i n t h e 400 nm r e g i o n as shown i n F i g . 1.

3

The luminescence band near 400 nm may be due t o a ' A ~ ~ -t r a n s i - ~ A ~ ~

t i o n . The i n i t i a l s t a t e o f t h i s t r a n s i t i o n i s a Stokes s h i f t e d upper l e v e l o f t h e t 2 e l e c t r o n c o n f i g u r a t i o n . The r e c i p r o c a l t r a n s i t i o n i n a b s o r p t i o n s!?o:?d be a broad band s h i f t e d t o h i g h e r energies.

However, i t i s d i f f i c u l t t o observe because i t i s b o t h s p i n and sym- metry f o r b i d d e n i n Oh symmetry and t h u s w i l l

be

a v e r y weak, broad band on t h e t a i l o f t h e a b s r o p t i o n edge. The a b s o r p t i o n edge shoulder a t h i g h e r e n e r g i e s i s p r o b a b l y due t o a charge t r a n s f e r t r a n s i t i o n . The f l u o r e s c e n c e l i f e t i m e o f t h e 700 nm emission band from

cr3+

i o n s i n t h e m i r r o r s i t e s i s t h e same a f t e r two-photon e x c i t a t i o n b u t t h e r i s e t i m e o f t h e f l u o r e s c e n c e i s measured t o be about 20 US

.

The

broadband emission around 400 nm has a decay t i m e o f about 8 H5 w i t h a

n e g l i g i b l e r i s e time. The measured f l u o r e s c e n c e l i f e t i m e o f t h e 400 nm band i s s i g n i f i c a n t l y s h o r t e r than expected f o r a s p i n - A-srbidden r a d i a t i v e t r a n s i t i o n which may i n d i c a t e quenching due t o

Fig. 1

-

Fluorescence band excited by two-photon absorption.

time of the 700 nm emission after two-photon e:icitatian whizh implies the presence of an indirect pamping process of the

normal

fl~icrescence transitions. If this is due to re1ar:at-ion S r o m t h e 400 n m le\~el, the rise time should be given b y

Here T~~ is the nonradiative decay time of the upper level while NE and N40* are the initial populations of the upper and lower levels, respectively. Using the measured fluorescence lifetimes with no direct pumping of the 'E level predicts a value of

30

Hs for tr. The slightly shorter observed value can be attributed to the presence of some pumping of the 'E level through one-photon absorption processes to the 4 ~ 2 g band. ~ e t e i n g - ~ , , ~ N ~ / N ~ ~ ~ = 1 7 . 9 4 J4s in Eq.

( 1 )

gives the observed result.

I 1 1

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SITE-SELECTION SPECTROSCOPY: EXCHANGE COUPLED

PAIRS

When the chromium ions are located close enough to each other to interact strongly, they produce satellite lines in the optical spectra called N-lines. Figure

2

shows the region of the fluorescence spec-- trum near the R lines for several different escitation wavelengthi in

this region. observed spectral lines can be attributed to transi-

tions between levels of exchange coupled pairs of zr3+ ions.

The interaction Hamiltonion describing the exchange coupling of the chromium ion pairs is He,=-J(Sl'S2) where the coupling constant J is positive for ferromagnet~c coup1 ing and negative for antif erromagnetic

coupling. The ground state energy level splitting due to ferromag-

netic exchange coupling for two cr3+ ions with spin 3/2 is

QE=O,J,3J,6J. For antiferromagnetic coupling of these ions the split- ting is QE=0,3J,SJ,6J. The spectra in Fig. 2 can be associated with the energy level diagrams of six different types of pairs, two are ferromagnetically coupled and four antiferromagnetically coupled.

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Fig. 2

-

Fluorescence s p e c t r a o f N - l i n e s f o r f o u r e x c i t a t i o n wave- l e n g t h s .

d i f f e r e n t p a i r s w i t h o u t e x t e n s i v e i n v e s t i g a t i o n u s i n g t o o l s such as e l e c t r o n pa.ramagnetic resonance and u n i a x i a l s t r e s s . However, t h e l a t t i c e s t r u c t u r e of a l e x a n d r i t e p r o v i d e s a v a r i e t y o f d i f f e r e n t combinations o f c l o s e neighbor p a i r s i t e s i n v o l v i n g b o t h m i r r o r and i n v e r s i o n l o c a t i o n s .

I V

-

FOUR-WAVE M I X I N G

Using t h e crossed beam technique described i n

/ 2 / ,

e x c i t e d s t a t e p o p u l a t i o n g r a t i n g s were e s t a b l i s h e d and probed independent1 y f o r

cr3+

i o n s i n m i r r o r and i n v e r s i o n s i t e s . F i g u r e 3 shows t h e g r a t i n g decay r a t e s p l o t t e d as a f u n c t i o n o f t h e c r o s s i n g angle o f t h e w r i t e beams,

8 , f o r t h r e e e x c i t a t i o n wavelengths. The s o l i d p o i n t s a r e t w i c e t h e values measured f o r t h e f l u o r e s c e n c e decay r a t e s o f m i r r o r s i t e and i n v e r s i o n s i t e ions. The f a c t t h a t t h e g r a t i n g decay r a t e s a r e inde- pendent a f 8 and approximately t h e same as t w i c e t h e f l u o r e s c e n c e decay r a t e s i n d i c a t e s t h a t t h e g r a t i n g decay i s due t o f l u o r e s c e n c e emission w i t h o u t t h e presence o f l o n g range energy m i g r a t i o n .

The probe beam s c a t t e r i n g e f f i c i e n c y was measured t o be about f o r

a l l t h r e e e x c i t a t i o n wavelengths. T h i s i s d i r e c t l y p r o p o r t i o n a l t o

t h e

square

0 4

t h e

modulation

depth

o#

t h e g r a t i n g . The l a t t e r can be associated w i t h t h e d i f f e r e n c e s i n e i t h e r t h e a b s o r p t i o n or d i s p e r s i o n p r o p e r t i e s o f t h e sample when t h e

cr3+

i o n s a r e i n t h e e x c i t e d s t a t e and t h e ground s t a t e . C a l c u l a t i o n s based on a simple t w o - l e v e l system model i n d i c a t e t h a t changes i n t h e a b s o r p t i o n c o e f f i c i e n t can n o t account f o r t h e observed d i f f r a c t i o n e f f i c i e n c i e s . Using a simple o s c i l l a t o r model f o r d i s p e r s i o n , t h e change i n t h e index o f r e f r a c t i o n between t h e peak and v a l l e y r e g i o n s o f a p o p u l a t i o n g r a t i n g can

be

approximated as

Fig. 3

-

Four-wave m i x i n g s i g n a l decay r a t e s f o r Ax=579.1 nm ( t r i a n g l e s )

;

488.0 nm ( c i r c l e s ) ; and 514.5 nm (squares).

s t a t e i n t h e peak r e g i o n o f t h e g r a t i n g . The sum r u n s over a l l

p o s s i b l e t r a n s i t i o n s . I f o n l y t h e low o s c i l l a t o r s t r e n g t h t r a n s i t i o n s between 3d l e v e l s a r e i n c l u d e d i n t h e sum i n Eq.

( 2 > ,

t h e t . h e o r s t i c a 1 p r e d i c t i o n i s s i g n i f i c a n t l y l e s s than t h e observed s c a t t e r i n g e f f i - ciency. However, if a h i g h o s c i l l a t e r s t r e n g t h charge t r a n s f e r t r a n - s i t i o n near t h e band edge i s i n c l u d e d i n t h e c a l c u l a t i o n , Eq. (2)

p r e d i c t s a v a l u e c o n s i s t e n t w i t h t h e experimental r e s u l t s .

V.

-

DISCUSSION CONCLUSIONS

The performance o f a l e x a n d r i t e as a t u n a b l e s o l i d s t a t e l a s e r m a t e r i a l i s c r i t i c a l l y dependent on t h e s p e c t r a l dynamics o+ t h e

cr3+

ions. The U.V. s p e c t r a l bands observed i n t h i s study can c o n t r i b u t e t o degradation o f l a s i n g o p e r a t i o n through e x c i t e d s t a t e a b s o r p t i o n and n o n l i n e a r o p t i c a l e f f e c t s . The s t r e n g t h o f t h e four-wave m i x i n g s i g n a l i n d i c a t e s t h a t t h i r d o r d e r n o n l i n e a r processes may be impor- t a n t . The s t r e n g t h of t h e two-photon a b s o r p t i o n i n d i c a t e s t h a t t h i ~ can be an i m p o r t a n t l o a s mechanism f o r pumping i n t h e 532 nm s p e c t r a l region. The s i t e - s e l e c t i o n s p e c t r o s c o ~ y r e s u l t s show t h e importance of h a v i n g a u n i f o r m d i s t r i b u t i o n o f C r i o n s i n t h e c r y s t a l t o m i n i - mize t h e presence o f exchange coupled p a i r s which do n o t c o n t r i b u t e t o l a s e r emission. I n a d d i t i o n ,

cr3+

i o n s i n i n v e r s i o n sites do n o t c o n t r i b u t e t o t h e l a s i n g and t h u s energy t r a n s f e r from t h e m i r r o r s i t e i o n s t o i n v e r s i o n s i t e i o n s a c t s as a l o s s mechanism.

ACKNOWLEDGMENTS. T h i s research was sponsored by t h e U.S. Army

Research O f f i c e .

REFERENCES

/ 1 / Walling, 3. C., Peterson, 0. G., Jenssen, H. F., M o r r i s , R. C. and

O ' D e l l , E. W., IEEE

J.

Buant. Elect.BE-16 (1980) 1302.

/2/

Ghatzawi, A. f l . , Tyminski,

3.

Kc,

Powell, R. C. and W a l l i n g , 3.